Many different types of infrared detectors typically rely on high thermal isolation from the surrounding environment to achieve a desired level of performance. For example, an infrared detector may be encapsulated within a vacuum package to minimize thermal conduction via gas molecules.
A microbolometer and a silicon-on-insulator (SOI) diode are examples of these types of infrared detectors, which may be used within infrared cameras (e.g., various types of infrared imaging devices). The microbolometer and the SOI diode are typically fabricated on monolithic silicon substrates to form an image detector array, with each infrared detector of the image detector array functioning as a pixel to produce a two-dimensional image. The change in resistance of each infrared detector is translated into a time-multiplexed electrical signal by circuitry known as the read out integrated circuit (ROIC). The combination of the ROIC and the image detector array is commonly known as a focal plane array (FPA) or infrared FPA (IRFPA).
As noted, a typical FPA is situated within a vacuum package (also known as a vacuum package assembly) to provide a high vacuum environment that is generally required to achieve a desired sensitivity from the infrared detectors. However due to various factors (e.g., outgassing and/or leaks), the vacuum integrity of the vacuum package assembly degrades over time, which results typically in a gradual loss of vacuum pressure and consequently a reduction in sensitivity of the infrared detectors and an overall reduction in performance of the infrared camera.
A conventional approach to determine the vacuum integrity of the vacuum package is to measure the FPA's responsivity to calibrated temperature sources placed in front of the infrared camera's lens. However, the calibrated temperature sources add to the cost and complexity of the test and this approach may be difficult to perform as a field diagnostic (i.e., outside of the lab environment). Furthermore, this approach may not be accurate if the infrared camera lens or other optical path components are degraded, which may be the true source of the infrared camera's performance degradation.
As a result, there is a need for improved techniques to measure vacuum pressure of a vacuum package, such as for a microbolometer FPA of an infrared camera.